Datasheet
TMC2300 DATASHEET (Rev. 1.02 / 2019-NOV-06) 41
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7 Fitting the Motor
Especially for low voltage operation, the motor should be carefully selected to give a good fit to the
application’s mechanics, as well as available supply voltage and current. Therefore, it is important to
understand the supply voltage requirement for a given motor. Both, the generation of a certain
torque, and the ability to provide this torque at a desired velocity, require a motor specific voltage.
These two components add up.
Main relevant parameters for a stepper motor:
Nominal (RMS) coil current I
COILNOM
[A]
Nominal coil resistance R
COIL
[Ω]
Rated coil voltage U
N
= R
COIL
* I
COILNOM
[V] (sometimes specified instead of I
COILNOM
)
Holding torque at I
COILNOM
HoldingTorque [Nm]
The specified motor torque is reached with the RMS I
COIL
current in both motor coils, in order to build
up the required magnetic field strength. A lower current will basically proportionally generate a lower
torque, e.g. 70% of torque at 70% current. Even a reduction to 70% saves a lot of energy, because
power dissipation goes with the square of the current. Thus, a motor with more reserves can offer
better efficiency!
With this, calculate the required supply voltage U
BAT
for motor stand still and slow motion, taking into
account the driver’s power stage resistance plus 0.3V loss in the sense resistor:
I
COIL
is the RMS motor current which gives the desired torque.
For higher velocity operation (more than a few electrical rotations per second), the motor specific
back EMF constant C
BEMF
should be additionally taken into account (see below explanation). With this,
the lowest feasible supply voltage for a given motor and a maximum velocity [RPM] calculates to:
Adapt your motor to battery operation
With most motor suppliers you have the chance to adapt the coil winding. This allows to trade in a
lower motor voltage for battery operation versus higher motor current. E.g. a motor with a short,
thick coil wire can work at a lower voltage, than the same motor with a long, thin coil wire, but it
needs a higher current for the same torque.
UNDERSTANDING THE BACK EMF CONSTANT OF A MOTOR
The back EMF constant is the voltage a motor generates when turned with a certain velocity. Often
motor datasheets do not specify this value, as it can be deducted from motor torque and coil current
rating. Within SI units, the back EMF constant C
BEMF
has the same numeric value as the torque
constant. For example, a motor with a torque constant of 1 Nm/A would have a C
BEMF
of 1V/rad/s.
Turning such a motor with 1 rps (1 rps = 1 revolution per second = 6.28 rad/s) generates a back EMF
voltage of 6.28V. Thus, the back EMF constant can be calculated as:
I
COILNOM
is the motor’s rated phase current for the specified holding torque
HoldingTorque is the motor specific holding torque, i.e. the torque reached at I
COILNOM
on both coils.
The torque unit is [Nm] where 1Nm = 100Ncm = 1000mNm.
The BEMF voltage is valid as RMS voltage per coil, thus the nominal current has a factor of 2 in this
formula.